Dental composites continue to be favored restorative materials in the anterior region, although their use is limited to single-unit restorations. In both anterior and posterior restorations, lifetimes are limited by two factors: polymerization shrinkage and mechanical failure. Although composite properties continue to improve, their toughness, durability, and strength remain inadequate, particularly when large restorations are considered. Wear is still a concern in approximal and occlusal contact areas. Studies related to the resin matrix component currently focus on the shrinkage problem; studies related to the filler component currently focus on alternative filler morphologies, panicle sizes, and filler composition. Relatively little in the dental community has been done concerning the filler-matrix interface beyond exploring silanation variables, although the structural composites community at large has recently focused on the concept of an engineered interphase as a means of improving composite properties. However, that community is primarily interested in fiber-reinforced, not particulate-reinforced composites. We propose to adapt this concept for the design of stronger and especially tougher dental composites in order to expand the indications for composite restorations. In designing new materials, it is useful to adopt a methodology that shortens development time and has predictive power. We have experienced success in implementing finite element (FE) modeling for predicting the modulus and toughness of particulate filled composites, and propose extending this approach to more complex composite systems in combination with an experimental approach that validates the FE models. Specifically, we propose the following aims: 1.) To fabricate, model, and predict toughening of matrix polymers containing ductile organic additives; our hypothesis is that the presence of such compounds (% elongation=400%) promotes plasticity and energy dissipation.; 2.) To fabricate, model, and predict toughening of composites with novel ductile interphases; our hypothesis is that the presence of the interphase provides additional plasticity and augments toughening by the crack-pinning mechanism; and 3.) To model the effect of filler particle size and shape on composite strength and toughness. The outcome of this work will be a blueprint for the rational design of longer-lived composites.